Fixed panic when suiteInfo.callerName does not contain the package

Hi, firstly thanks for creating this! I found it quite useful for setting up BDD in Go.

I found an issue when using the BDD formatter, closely related to issue #5.

The callerName can be in two formats: 

    com/remogatto/prettytest.(*testSuite).Should_be_cool

or 

    (*testSuite).Should_be_cool

Currently only the first case was handled and the second caused a panic in `BDDFormatter.splitString`:

```
BDD Formatter:
panic: runtime error: index out of range [recovered]
	panic: runtime error: index out of range

goroutine 4 [running]:
testing._func_003(0xb7594fd4, 0xb7594100, 0xb7594fdc)
	/usr/lib/go/src/pkg/testing/testing.go:268 +0x11f
----- stack segment boundary -----
_/home/hari/prog/prettytest.(*BDDFormatter).splitString(0x18666ce0, 0x1864e2c0, 0x39, 0x8100f0c, 0x1, ...)
	/home/hari/prog/prettytest/prettytest.go:264 +0xa4
```

I've attached commits which provide support for both callerName formats.

Thanks!
pranav